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Re-sizing and Resolution.

The question is about how to resize a map to 4mb ( to upload here in CG) without lose a lots of resolution.

Here i have 2 samples about i am trying to say. I have a map with a nice resolution to read and see the details, but when i try set it to 4 mb i lose a lots of quality.
See my legend with a cool quality and my whole map getting all messed with my colors

When saving, you could try to reduce the image quality. That way, you can fit a bigger image inside the 4.6 mb limit. From what I have seen, it won't decrease the image quality as much as sqeezing it.
Another thing is to try the diffreent resampling options when compressing the image (with photoshop for example). It might help a bit too.

9000*4800px is very large, perhaps you could consider making it a little smaller. Looking at your text, I think that it could still be legible, albeit quite small, at 75% or even 50% of the size. At half that size you should be able to get within the size limit with a good quality (like 80% or more using .jpg).
Alternatively, split the image. Since practically no one has a screen large enough to show such resolution, it will be seen only in parts anyway.

I made both ways.
The sample here i made a JPG and resize (75% from the original). In both ways, after resize i have to lower the quality (lvl 6 in Photoshop "Save As" JPG), after this i guess resizing a JPG saved at max quality, it looks better.
I tried PDF too, not a good resolution.

thanks a lots trying to help.
Photoshop have a automatic tool to split the image?

I don't know if PS has an automatic tool to split the image, but it has the crop tool which can be used much in the same way (crop-export; crop another part-export etc).
I believe that if you resize first and only after export to .jpg (or whatever) you should obtain a better result than if you first export to .jpg and then resize.
Also, after resizing, instead of "save as" try using "save for web and devices" (or whatever it is in English), that for some reason seems to produce smaller images even at the same quality. You may get a message saying that the image is very large and there may be slowdowns or something, but it should still work.